Since the development of integrated circuit technology, computers and computer storage devices have been made from integrated circuit (IC) chips formed from wafers of semiconductor material. After a wafer is made, the circuits are typically separated from each other by dicing the wafer into small chips. Thereafter, the individual chips are bonded to carriers of various types, interconnected by wires and packaged. Such "two-dimensional" packages of chips fail to optimize the number of circuits that may be fabricated in a given space, and also introduce undesirable signal delays, capacitance, and inductance as signals travel between chips. Recently, three-dimensional arrays of chips have emerged as an important packaging approach. A typical multichip electronic module consists of multiple IC chips adhesively secured together as a monolithic structure (a "stack") .
One issue to be addressed is the issue of thermal expansivity of the materials of the stack. Different materials have different rates of thermal expansion associated therewith. For example, in an electronic module, when the IC chips and the adhesive layers are made from different materials, each will expand at a different rate when subjected to temperature increases. Unfortunately, electronic modules typically heat up during operation due to ohmic losses. Thus, when the temperature of a module increases, the chips and adhesive layers (bonding the chips together) expand at different rates inducing stress within the module.
Further module stress is induced when module expansion is constrained. For example, if a side surface of an electronic module is solder bump bonded to a mounting substrate, the bonds will constrain the expansion of the module in a direction perpendicular to the main surfaces of the chips comprising the module (hereinafter referred to as the "Z direction"). This constraint arises from, for example, each IC chip being bonded to the mounting substrate such that it is unable to move in the Z direction. Thus, the forces of the thermally expanding adhesive are constrained by the immobile substrate. Due to a generally high yield point, the "compressed" adhesive maintains its bonding and causes physical stress on the module which can lead to damage and failure of the solder bumps. The solder bumps fracture as a result of the expansion and contraction of the electronic module caused by changes in temperature. This is a well understood phenomenon in the industry. The present invention is directed towards solving these problems.